This invention relates to bismuth coating protection for copper-containing surfaces and is more particularly concerned with improved compositions and methods for providing protective bismuth metal platings on such surfaces.
The present invention constitutes a further development from the invention described in commonly assigned International Patent Application No. PCT/US95/03574, published under publication No. WO 95/25008 and incorporated herein by reference.
Briefly stated, the prior application describes compositions, and methods of use, for immersion plating bismuth metal onto copper-containing surfaces, such as copper surfaces of printed circuit boards (copper pads for receiving surface-mount electronic components, copper-plated through holes for receiving the leads of pin-in-hole type electronic components, etc.). Immersion plating is a well known process in which a dissolved metal is displaced out of solution by a more active (less noble) base metal from a workpiece surface that is contacted with a plating medium containing the dissolved metal. Displacement occurs due to a difference between the electropotentials of the plating metal and the base metal, with the displaced metal depositing on and coating the workpiece surface. The workpiece may be contacted with the plating medium by any suitable contacting method, such as dipping, spraying, or flood coating.
Immersion-plated bismuth metal coatings provide excellent protection of copper-containing surfaces against tarnishing, corrosion, and other agents that can impair surface quality. As a protectant for copper surfaces of printed circuit boards, for example, immersion-plated bismuth metal coatings have been found to constitute an effective means of preserving solderability prior to and during processing of the boards for circuit assembly. They also exhibit a high surface energy which promotes wetting during soldering, and a high degree of flatness which is desirable for mounting surface-mount electronic components. As contrasted with lead-tin protective coatings commonly used in the electronics industry, protective bismuth metal coatings are not subject to degradation by the formation of intermetallics with copper. Nor do they exhibit the problems of oxidation, volitization on heating, and poor solderability (especially poor hole-fill for wave soldering) that typify less widely used organic protective coatings.
The coating compositions of the prior application are acidic aqueous systems containing dissolved bismuth and halide, such as chloride or bromide. The bismuth is preferably provided as a constituent of a bismuth salt, and the halide as a constituent of the acid (i.e., a halogen acid). The acid solubilizes the bismuth salt, and the halide shifts the relative electropotentials of the bismuth and copper to enable the plating reaction to proceed.
In a preferred mode, the invention of the prior application also utilizes an iodide, such as an iodide salt or an organic iodide, as a complexing agent to promote adhesion of the bismuth metal coating to the surface being plated. The iodide may be provided as an ingredient of the plating bath, or as part of a pre-plating bath with which the surface is treated prior to immersion plating.